sunpy HelioviewerClient is deprecated

[HelioviewerClient]( https://docs.sunpy.org/en/v5.1.4/generated/api/sunpy.net.helioviewer.HelioviewerClient.html) has to be replaced with [hvpy](https://hvpy.readthedocs.io/en/stable/) to be compatible with sunpy 5.2.0 and above. For now, I will pin the sunpy version to be <5.2.0.